Recently, prepaid cards which store information of prepaid money have been used in many fields, e.g., vending machines, traffic toll collecting machines, etc. These machines for dealing such prepaid cards generally accommodate an image recording apparatus which record images such as characters and/or figures on the prepaid card for visualizing information of the balance registered therein.
Conventionally, several recording systems, such as a magnetic recording system, a thermal recording system, a mechanical punch recording system, etc. are used for the image recording apparatus to the prepaid card. Recently, however, the thermal image recording apparatus has been increasingly used from reasons of easiness and convenience for use. To a reliability for use of the prepaid cards, these image recording apparatus including the thermal image recording apparatus must have a sufficient durability against various stimulative causes such as light, heat, magnetic flux, mechanical wear, etc. from the external environmental conditions.
In the thermal image recording apparatus, the prepaid cards are provided with a metal film as an image recording layer from a reason of security. At present, a tin film is widely used for the prepaid cards. The tin has approximately a melting point 236.degree. C.
In a recording operation for the metal film such as the tin film, a thermal head of the thermal image recording apparatus heats portions of the metal film over the melting point so that the melted portions are dissipated out from the prepaid cards. Generally the metal film such as the tin film has a relatively low sensitivity in comparison to thermosensitive ink which is adopted in thermosensitive recording paper and the like. Thus the thermal image recording apparatus for prepaid cards require high recording power for obtaining images comparable to the thermosensitive recording paper.
When recording images continuously on the metal film, a temperature of the thermal head varies at a start and an end of the recording operation, because a heat accumulated in the thermal head is gradually dissipated. Thus, for example, it is often difficult to keep a uniform density in all marks image when they are recorded.
Further, if it is attempted to record one dot mark at a location widely spaced from adjacent past and future marks, the recording of one dot mark would be failed because the temperature of the heating resistor could not instantly exceed a threshold level of temperature required for melting the metal film.
In order to prevent such defects, a thermal image recording apparatus incorporating a heat accumulation control for the thermal head has been disclosed.
An example of a system of conventional heat accumulation controls is disclosed in the Japanese Patent Disclosure (Kokai) No. 61-15469. This heat accumulation control system disclosed in the Japanese Patent Application relates to image recording to the thermosensitive recording paper, but not to metal recording media such as prepaied cards.
In this heat accumulation control system adopted in the conventional thermal image recording apparatus, a heating energy required for recording images on a particular line is calculated in reference to information of recorded images on one or more past lines. Then a required pulse width of heating current supplied to the thermal head is determined.
This conventional heat accumulation control system is advantageous to the image recording operation for the thermosensitive recording paper, but is not sufficient for the metal recording media. That is, the conventional heat accumulation control system is adapted to control energy for recording medium with a relatively low threshold level of printing image. For example, the thermosensitive recording paper and a thermosensitive ink ribbon for thermal printers have a threshold image printing level of about 60.degree. to 80.degree. C.
For example, no heating current is supplied to the present recording area if space data (i.e., data not causing of image printing) continue long in that area, while heating current is applied for a longer time to mark data (i.e., data for causing of printing image) appearing in following such a long space data in comparison to a mark data within a continuing mark data.
However, if this conventional heat accumulation control system were applied to low sensitivity image recording media such as metal films, it is necessary to extend extremely longer the time for applying a recording current to the heating resistor.
Therefore, when shifting from continuous space data to mark data, a recording current applied to the heating resistor and a thermal stress caused by the recording current disadvantageously exceed respective tolerance limits for the heating resistor. This results that the thermal head would be worn out rapidly.